I use Mail.app's mailbox search (Command-Option-F) extensively, but it always bothered me that in order to cancel the search results and look at the unfiltered selected mailbox, I had to grab the mouse to click the Cancel button in the search bar.

It turns out you can do this via the keyboard; it's just not as straightforward and obvious as it should be (like Command-Period or Escape): Hit Command-Option-F again, then hit Delete or Backspace. Presto, the search results vanish.

OK, just last night, around 10PM central I was searching osxhints and google to try and find a way to do exactly this. I was also always bothered by having to take my hands off the keyboard to end the search.